This book intended to understand why and how new defence collaborations in Europe are created and argued that the driving force for this is the subregional dimension. However, these subregional dynamics are less visible for the public and are studied less by scholars compared to NATO and EU-level dynamics. Thus, the subregional level regarding military cooperation – the processes going on below the large European organizations of EU and NATO – is often neglected even though most of the practical work is happening there. On the subregional level, European armed forces organize multinational exercises, cooperate on weapons development and procurement, establish multinational formations, train together, pool and share their capabilities and so on. Furthermore, the negotiations about these projects also take place on the subregional level. Thus, not surprisingly, the extensive cooperation of European militaries on the subregional level provides the foundation of European-level (NATO/ EU) defence collaborations. These subregional processes shape European-level dynamics but European-level processes also have an impact on subregional defence matters.

To understand the complexities of this issue, the book offered three major innovations. First, the book proposed a theoretical framework of the subregional approach and applied it to three cases: the Lancaster House Treaties, the Nordic Defence Cooperation and the Central European Defence Cooperation. The theoretical framework of the subregional approach suggests that both ‘structural’ and ‘situational’ factors are needed to create a new defence cooperation, and these factors interact with each other in a particular way. Second, the book argued that, instead of focusing on states, governments or international organizations, the best way to grasp the essence of defence cooperation is using defence policy communities (DPCs) as the unit of analysis. Finally, throughout the book, it was demonstrated that a multidisciplinary approach was needed for studying defence collaborations in order to be able to analyse the different aspects of this phenomenon.

The proposed theoretical framework of the subregional approach suggests that both long-term processes and rapidly changing events play a role when a multinational defence cooperation (MDC) is established. Thus, the book distinguishes between ‘structural’ and ‘situational’ factors. The ‘structural’ factors are those elements that have been evolving for a long time and change slowly.